To induce antigen-specific T cell activation and clonal expansion, two signals provided by antigen-presenting cells (APCs) must be delivered to the surface of resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med 165, 302-319; Mueller, D. L., et al. (1990) J. Immunol. 144, 3701-3709; Williams, I. R. and Unanue, E. R. (1990) J. Immunol. 145, 85-93). The first signal, which confers specificity to the immune response, is mediated via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Schwartz, R. H. (1990) Science 248, 1349-1356). Costimulation is neither antigen-specific, nor MHC restricted and is thought to be provided by one or more distinct cells surface molecules expressed by APCs (Jenkins, M. K., et al. (1988) J. Immunol. 140, 3324-3330; Linsley, P.S., et al. (1991) J. Exp. Med 173, 721-730; Gimmi, C. D., et al., (1991) Proc. Natl. Acad Sci. USA. 88, 6575-6579; Young, J. W., et al. (1992) J. Clin. Invest. 90, 229-237; Koulova, L., et al. (1991) J Exp. Med 173, 759-762; Reiser, H., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 271-275; van-Seventer, G. A., et al. (1990) J. Immunol. 144, 4579-4586; LaSalle, J. M., et al., (1991) J. Immunol. 147, 774-80; Dustin, M. I., et al., (1989) J. Exp. Med. 169, 503; Armitage, R. J., et al. (1992) Nature 357, 80-82; Liu, Y., et al. (1992) J. Fxp. Med. 175, 437-445).
Considerable evidence suggests that the B7 protein, expressed on APCs, is one such critical costimulatory molecule (Linsley, P. S., et al., (1991) J. Exp. Med 173, 721-730; Gimmi, C. D., et al., (1991) Proc. Natl. Acad Sci. USA. 88, 6575-6579; Koulova, L., et al., (1991) J. Exp. Med. 173, 759-762; Reiser, H., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 271-275; Linsley, P. S. et al. (1990) Proc. Natl. Acad. Sci. USA. 87, 5031-5035; Freeman, G. J. et al. (1991) J. Exp. Med 174,625-631.). B7 is the counter-receptor for two ligands expressed on T lymphocytes. The first ligand, termed CD28, is constitutively expressed on resting T cells and increases after activation. After signaling through the T cell receptor, ligation of CD28 induces T cells to proliferate and secrete IL-2 (Linsley, P. S., et al. (1991) J. Exp. Med. 173, 721-730; Gimmi, C. D., et al. (1991) Proc. Natl. Acad. Sci. USA. 88, 6575-6579; Thompson, C. B., et al. (1989) Proc. Natl. Acad. Sci. USA. 86, 1333-1337; June, C. H., et al. (1990) Immunol. Today. 11, 211-6; Harding, F. A., et al. (1992) Nature. 356, 607-609.). The second ligand, termed CTLA4 is highly homologous to CD28 but is not expressed on resting T cells and appears following T cell activation (Brunet, J. F., et al., (1987) Nature 328, 267-270). Although B7 has a higher affinity for CTLA4 than for CD28 (Linsley, P. S., et al., (1991) J. Exp. Med. 174, 561-569), its function is still unknown. The importance of the B7:CD28/CTLA4 costimulatory pathway has been demonstrated in vitro and in several in vivo model systems. Blockade of this costimulatory pathway results in the development of antigen specific tolerance in murine and humans systems (Harding, F. A., et al. (1992) Nature. 356, 607-609; Lenschow, D. J., et al. (1992) Science. 257, 789-792; Turka, L. A., et al. (1992) Proc. Natl. Acad. Sci. USA. 89, 11102-11105; Gimmi, C. D., et al. (1993) Proc. Natl. Acad. Sci USA (90 6586-6590); Boussiotis, V., et al J. Exp. Med. (accepted for publication)). Conversely, expression of B7 by B7 negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L., et al. (1992) Cell 71, 1093-1102; Townsend, S. E. and Allison, J. P. (1993) Science 259, 368-370; Baskar, S., et al. (1993) Proc. Natl. Acad. Sci. 90, 5687-5690.). Therefore, manipulation of the B7:CD28/CTLA4 pathway offers great potential to stimulate or suppress immune responses in humans.